Signals are often transmitted between contacts of an integrated circuit device, such as between pin on integrated circuits (ICs) of a circuit board for example, or between contacts on connectors that provide an interface to enable the transmission of signals between two devices. Many interfaces use 50 ohm impedances as it is an optimal impedance for transfer of power. In order to achieve a constant impedance all the way from an on-die transmitter to the on-die receive circuitry of another integrated circuit, it is necessary to implement a 50 ohm impedance on the die for both the transmit and receive circuitry. This 50 ohm impedance would typically be a resistance to a power supply or to a DC voltage generated on-die.
On-die resistors have approximately +/−20% process variation, which means that a simple resistor is not sufficiently accurate to achieve the 50 ohm impedance. A commonly used design includes a trimmable resistor and a trimming circuit. The trimming circuit determines what trim legs should be enabled to trim the resistance to achieve 50 Ohm, and therefore remove the +/−20% process uncertainty. However, a trimming circuit can typically only measure impedance at DC, but not at high frequencies. Further, a limitation of this design is the trim legs that remain off. While these legs should be high impedance, the parasitics of a switch such as a transistor associated with the trim legs make the impedance look smaller at high frequencies (e.g. greater than 1 GHz). That is, it looks like all switches are enabled and therefore all of the trim legs are on at high frequencies, reducing the impedance which results in amplitude reduction and therefore limiting the bandwidth of a termination block.
Accordingly, methods and circuits that extend the bandwidth of a termination block are beneficial.